First of all, T5's are every bit as powerful as metal halides. My 8 x 54 watt fixture was putting out the same PAR on the sand bed as a 400 watt halide, and the spread was double. So, in that sense you could say that T5's are stronger than halides, watt for watt. There are more variables, but anyway....I think the hydnophora died because of nutrient rich waters. Just like Dave said.
Also, montipora capricornis isn't a high light sps
IME. I have montis under a dimmable fixture just like yours with no white and just the blues on at 100% and they grow just fine.
Half and half should be good for the sps